1. Field of the Invention
This invention relates to a light emitting device based on a plurality of light emitting diodes (LEDs). In particular, although not exclusively, the invention concerns an alternating current (AC) driven light emitting device that can be operated from a high voltage (110/220V) supply. Moreover, the invention concerns AC light sources based on a plurality of such light emitting devices.
2. Description of the Related Art
As is known LEDs are intrinsically direct current (DC) devices that will only pass an electrical current in a single direction and have traditionally been driven by low voltage (e.g. 3.5V for gallium nitride LEDs) DC sources.
White light generating LEDs, “white LEDs”, are a relatively recent innovation and offer the potential for a whole new generation of energy efficient lighting systems to come into existence. It is predicted that white LEDs could replace incandescent, fluorescent and compact fluorescent light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor to provide light which appears to the human eye as being nearly white in color.
In lighting applications it is desirable to be able to operate white LEDs directly from a high voltage (110/250V) AC mains power supply without the need for expensive power supplies and driver circuitry. US 2004/0080941 discloses a single-chip integrated LED that is for direct use with a high voltage (110V/220V) AC power supply that comprises two arrays (strings) of series-connected individual LEDs. The strings are connected in parallel with the LEDs in opposite polarity in a half-wave rectifier configuration such that the LEDs are self-rectifying. A sufficient number of LEDs (e.g. twenty eight per string for 110V operation and fifty five per string for 220V operation) is provided in each string to drop the total source voltage across the LEDs. During the positive half of the AC cycle one string of LEDs is forward biased and energized, while the other string is reverse biased. During the negative half of the AC cycle, the other string of LEDs is forward biased and energized, while the first string is reverse biased and not energized. Thus the strings are alternately energized at the frequency of the AC supply (50-60 Hz) and the single-chip LED appears to be constantly energized. The single-chip LED is formed by epitaxially depositing layers of n-type semiconductor material, optically active layers and p-type semiconductor material in succession to define individual LEDs on a single wafer. Adjacent LEDs are interconnected by depositing conducting layers between individual LEDs. Typically individual LEDs are spaced on the LED wafer at a 20 μm separation. Although such a fabrication is compact it has a disadvantage that since only one LED string is energized at a time the arrangement has only a 50% payload.
US 2007/0273299 teaches an AC LED package in which pairs of LEDs or series-connected strings of LEDs are connected in an opposing parallel configuration with at least one capacitor connected in series with each parallel configuration and the AC power supply. The LEDs and capacitor(s) can be fabricated as a single chip, a single package, an assembly or a module. The capacitor regulates the amount of current and forward voltage delivered to the one or more opposing parallel LEDs based on the voltage and frequency provided by the AC driver. Since only LED strings with the same polarity are energized at any one time the arrangement has only a 50% payload.
Presently AC LEDs can be fabricated as a single-chip device as for example is taught in US 2004/0080941, from a plurality of interconnected discrete packaged LEDs or from a plurality of interconnected individual surface mounted devices (SMDs). In the case of a single-chip device the LEDs are monolithically fabricated on a single wafer and LEDs connected in series by photo lithographic deposition of conductors between LEDs. Whilst such an arrangement can achieve a very high packing density of LEDs per unit area of wafer (of the order of 100 to 400 per square cm (cm2) depending on the size of the LED chip) it has a fundamental drawback in that during fabrication a failure of a single LED means that the entire device has to be discarded resulting in a high production cost. In contrast AC LEDs constructed by interconnecting discrete packaged devices, in particular white LEDs that include a phosphor material for wavelength conversion, results in a very low packing density of the order of 5 per cm2. It is also known to fabricate AC LEDs by mounting, typically by soldering, individual SMD LEDs onto a metal core printed circuit board (MCPCB) and such an arrangement can achieve packing densities of the order of 10 LEDs per cm2. However, such an arrangement is generally not suited to white LEDs which additionally require a layer of wavelength converting phosphor over the light emitting face of each LED.
The present invention arose in an endeavor to provide an AC light emitting device which at least in part overcomes the limitations of the known arrangements.